Food and drink products are often packaged in composite containers of the type having a tubular container body whose wall derives its structural strength from one or more paperboard plies spirally or convolutely wound about the tube axis and adhered together. The container body generally includes a moisture-impervious liner adhered to the inner surface of the paperboard body wall. The liner can be of various constructions, in some cases having a foil layer serving as the primary barrier of the liner, in other cases being made up entirely of polymers without any foil layer. The invention relates particularly to those containers having non-foil-based liners, also sometimes referred to as polymer liners. The container body at a top end has a rolled bead formed by rolling the tubular end of the container body outwardly and then downwardly. A membrane lid or closure is adhered to the end surface of the bead to hermetically seal the top end of the container closed. A removable and replaceable overcap generally is placed over the membrane and engages the bead in a snap-fit fashion so that when the container is initially opened by peeling off the membrane, the container can be resealed by replacing the overcap. Such containers are used for packaging a variety of food products.
To maintain product freshness until the package reaches the consumer, it is important for the hermetic seal between the membrane and the bead to remain intact during shipment and storage of the package. In some circumstances, the membrane can be subjected to internal pressure within the container as a result of temperature change and/or altitude change. For example, if the package is sealed at sea level and then transported to a substantially higher altitude or elevation, there will be a pressure on the membrane acting outwardly so as to tend to lift the membrane away from the bead; elevated temperature has a similar effect. The membrane must be able to withstand such pressures without the hermetic seal being compromised.
At the same time, it is desirable for the consumer to be able to peel the membrane off the bead with relatively little force. Additionally, the membrane should peel cleanly from the bead.
The above requirements tend to be in opposition with one another. Thus, achieving a strong seal that can withstand internal pressures generally tends to require a higher peel force to remove the membrane, and is more likely to result in tearing of the polymer barrier layer of the container liner upon opening.
The most commonly used polymer for obtaining a secure bond between the membrane and bead is SURLYN®, which is an ethylene acid copolymer having acid groups partially neutralized with zinc or sodium ions. SURLYN® bonds securely to itself. Typically a layer of SURLYN® is provided on the liner as well as on the membrane closure, and the two SURLYN® layers are heat-sealed together to attach the membrane to the bead of the container.
SURLYN® presents a number of benefits in high-speed commercial manufacture of composite containers. In particular, it permits a relatively higher “margin of error” during variations in manufacturing that will typically occur, relative to other adhesive systems. The problem with SURLYN® is that while it bonds extremely well, it can also be difficult to open (i.e., it bonds too well).
The above-noted problems are further exacerbated where the liner includes a polymeric barrier layer, as opposed to the more conventional foil layer. The foil layer tends to be frangible and will readily yield upon the application of the force by the consumer in opening the container. A polymeric barrier layer, on the other hand, has inherent resilience so that as the membrane is removed, the polymeric layer tends to stretch and tear in a manner that can leave undesired strings of material.
As a result, it is generally proposed to use alternative adhesive systems other than SURLYN®. For example, U.S. Pat. No. 5,979,748 describes the use of heat-flowable polymers such as high or low density polyethylene, metallocenes, and mixtures thereof, as the seal material on the bead, and polymers such as ethylene vinyl acetate, polyethylene, ethyl methyl acrylate, metallocenes, and mixtures thereof, as the seal material on the membrane. This adhesive system is designed to facilitate fracture of the adhesive system itself between the membrane and the liner on the bead. See also Elias U.S. Pat. No. 4,280,653, which describes the use of a heat-sealable co-extruded film laminate joined to the foil liner of the container for bonding the membrane to the bead. The laminate is designed to fail between its layers when the membrane is peeled off. The two layers of the laminate are dissimilar polymers. Specifically, the layer immediately adjacent the foil liner is polypropylene, and the other layer is a blend of polyethylene and ethyl methyl acrylate. The bond between these layer is weaker than the bond between the polypropylene layer and the foil layer, and weaker than the bond between the polyethylene-ethyl methyl acrylate layer and the membrane, such that these layers separate when the membrane is peeled off.
However, as noted, SURLYN® offers significant benefits as a sealant and hence it would be desirable to be able to use this sealant while overcoming the previously noted difficulties associated with its use in a membrane/bead closure system.